The present invention relates to a fuel supply system and a fuel supply method for an exhaust purifying catalyst device in an internal combustion engine. Specifically, the engine is provided with a plurality of cylinders, an exhaust manifold is connected to an exhaust port in each of the cylinders, an exhaust passage is connected to the exhaust manifold, and the catalyst device is provided in the middle of the exhaust passage.
Conventionally, in order to reduce NOx (nitrogen oxide) included in the exhaust gas, there has been taken a countermeasure that an exhaust gas recirculation device (hereinafter, refer to as an EGR device) is installed in an internal combustion engine. In accordance with the countermeasure mentioned above, it is possible to reduce the generation of NOx, however, it has not been yet achieved to do away with the generation of NOx, and NOx is still included in the exhaust gas. Accordingly, a NOx reduction catalyst reducing NOx in the exhaust gas has been developed in recent years. The exhaust purifying catalyst device provided with the NOx reduction catalyst is used together with the EGR device mentioned above, whereby an amount of NOx discharged to the atmospheric air from the engine is further reduced.
In the internal combustion engine having the catalyst device provided with the NOx reduction catalyst as mentioned above, it is necessary to execute the following matters in order to maintain the reduction efficiency of NOx. That is, a NOx reduction capacity of the catalyst is recovered by making an air-fuel ratio of the exhaust gas flowing into the exhaust passage rich at a predetermined timing before a NOx occlusion capacity of the catalyst is saturated, and supplying a reducing agent to the exhaust gas. For example, Japanese Laid-Open Patent Publication No. 2001-280125 and Japanese Laid-Open Patent Publication No. 2003-201836 propose supplying (adding) a fuel as a reducing agent to the exhaust gas.
In a four-cylinder engine in Japanese Laid-Open Patent Publication No. 2001-280125, in order to prevent the fuel serving as the reducing agent added to an exhaust system from circulating in an intake system via an exhaust gas recirculation device, a position of an adding port of a fuel adding nozzle adding the reducing agent (the fuel) and a position of an exhaust gas suction port of the exhaust gas recirculation device are specified. Specifically, an exhaust manifold connected to an exhaust port in each of the cylinders is connected to an exhaust pipe by an exhaust gas collecting pipe. An upstream end of the exhaust gas colleting pipe is connected to one end of the exhaust manifold. The adding port of the fuel adding nozzle is attached so as to face to the exhaust port of the fourth cylinder which is close to the one end of the exhaust manifold. The exhaust gas suction port is provided in the other end of the exhaust manifold. A timing for adding the fuel from the fuel adding nozzle is synchronized with a valve opening timing of the exhaust valve of the fourth cylinder.
Further, Japanese Laid-Open Patent Publication No. 2003-201836 discloses an example in which the fuel adding nozzle adding the fuel serving as the reducing agent is arranged in adjacent to the fourth cylinder in the four-cylinder engine. In the case that the third cylinder is in an exhaust stroke, an exhaust stream toward a distal end of the fuel adding nozzle, that is, a blow-back of the engine exhaust gas is generated within the exhaust port of the fourth cylinder due to a pressure pulsation. Japanese Laid-Open Patent Publication No. 2003-201836 discloses making a pause of the fuel injection from the fuel adding nozzle in the case that the exhaust stream in an opposite direction to the direction of the fuel injected from the fuel adding nozzle is generated. Accordingly, in the four-cylinder engine, it is possible to add fuel to the exhaust gas continuously for a crank angle of 540 degrees to the maximum. In this case, the 540 degrees is determined by subtracting 180 degrees corresponding to an exhaust stroke from a crank angle 720 degrees corresponding to one cycle (intake, compression, expansion and exhaust) of the third cylinder.
The inventors of the present invention made a study of a case that the fuel was added as the reducing agent to each of a left exhaust manifold connected to a left bank and a right exhaust manifold connected to a right bank in an eight-cylinder engine having the left bank and the right bank. The inventors found out that it is not necessary to avoid the back flow of the added fuel caused by the blow-back of the engine exhaust gas by appropriately setting an installed position of the fuel adding nozzle in the 8-cylinder engine, as is different from the existing four-cylinder engine. However, in the case of setting an injecting order to the eight cylinders to a general one, paying attention to the exhaust gas by the four cylinders in one bank, it was confirmed that there was generated a non-exhaust period in which all of the four cylinders did not become in the exhaust stroke. The non-exhaust period means a stroke in which the exhaust gas does not flow out to the exhaust manifold from any one of the four cylinders in the bank and the exhaust gas does not flow out to the exhaust passage from the exhaust manifold by extension. If the fuel is added, that is, injected to the exhaust manifold in the non-exhaust period, the added fuel does not get on the exhaust stream, and the added fuel collects on the exhaust system, for example, the wall surface of the exhaust manifold. Accordingly, considering one bank, the period in which the fuel serving as the reducing agent can be continuously added while avoiding the non-exhaust period comes to the crank angle of 450 degrees to the maximum. That is, it becomes shorter than the continuous crank angle of 540 degrees in the case of the four-cylinder engine.